1. Field of the Invention
The present invention generally relates to lithography, and more particularly to inspection of reticles used for lithography.
2. Background Art
Lithography is widely recognized as a key process in manufacturing integrated circuits (ICs) as well as other devices and/or structures. A lithographic apparatus is a machine, used during lithography, which applies a desired pattern onto a substrate, such as onto a target portion of the substrate. During manufacture of ICs with a lithographic apparatus, a patterning device (which is alternatively referred to as a mask or a reticle) generates a circuit pattern to be formed on an individual layer in an IC. This pattern may be transferred onto the target portion (e.g., comprising part of, one, or several dies) on the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate contains a network of adjacent target portions that are successively patterned. Manufacturing different layers of the IC often requires imaging different patterns on different layers with different reticles. Therefore, reticles must be changed during the lithographic process.
Current lithography systems project mask pattern features that are extremely small. Dust or extraneous particulate matter appearing on the surface of the reticle can adversely affect the resulting product. Any particulate matter that deposits on the reticle before or during a lithographic process is likely to distort features in the pattern being projected onto a substrate. Therefore, the smaller the feature size, the smaller the size of particles critical to eliminate from the reticle.
A pellicle is often used with a reticle. A pellicle is a thin transparent layer that may be stretched over a frame above the surface of a reticle. Pellicles are used to block particles from reaching the patterned side of a reticle surface. Although particles on the pellicle surface are out of the focal plane and should not form an image on the wafer being exposed, it is still preferable to keep the pellicle surfaces as particle-free as possible. For certain types of lithography (e.g., extreme ultraviolet (EUV) lithography), however, pellicles are not used. Because the EUV reticles are not covered, they are prone to particle contamination, which may cause defects in a lithographic process. Particles on EUV reticles are one of the main sources of imaging defects. Inspection and cleaning of an EUV reticle before moving the reticle to an exposure position can be an important aspect of a reticle handling process. Reticles are typically cleaned when contamination is suspected, as a result of inspection, or on the basis of historical statistics.
Reticles are typically inspected for defects using laser scanning scatterometer or imaging systems that use scattered light techniques. With this technique, a laser beam is focused on a reticle and a radiation beam that is scattered away from a specular reflection direction is detected. Particles on an object surface will randomly scatter the light. By observing the illuminated surface with a microscope, the particles will light up as bright spots. The intensity of the spots is a measure of the size of the particle.
A scatterometer operating with visible or ultraviolet (UV) light allows significantly faster reticle inspection than scanning imaging systems (e.g., confocal, EUV or electron beam microscope systems). There are known scatterometers that use a laser illumination beam and a coherent optical system with a Fourier filter in the pupil plane that blocks light diffracted from a pattern on the reticle. This type of scatterometer detects light scattered by particles over the level of background coming from a periodic pattern on the reticle.
One example of such a system is described in U.S. Pat. Application Publication No. 2007/0258086 A1 to Bleeker et al., published on Nov. 8, 2007, and entitled, “Inspection Method and Apparatus Using Same.” As shown in FIG. 1, an exemplary inspection system 100 includes a channel 102 including a microscope objective 104, a pupil filter 106, a projection optical system 108, and detector 110. A radiation (e.g., laser) beam 112 illuminates an object (e.g., a reticle) 114. Pupil filter 106 is used to block optical scattering due to the pattern of object 114. A computer 116 can be used to control the filtering of pupil filter 106 based on the pattern of object 114. Accordingly, filter 106 is provided as a spatial filter in a pupil plane relative to object 114 and is associated with the patterned structure of object 114 so as to filter out radiation from the scattered radiation. Detector 110 detects a fraction of radiation that is transmitted by filter 106 for detection of contamination particles.
It is not feasible, however, to use an inspection system such as inspection system 100 on reticles having arbitrary (i.e., non-periodic) patterns. This limitation is a result of saturation of the detector by light diffracted by the pattern. The detector has limited dynamic range and cannot detect light from a particle in the presence of light scattered from the pattern. In other words, correspondent light can be efficiently filtered out by a spatial filter in a Fourier plane of a coherent optical system only for a periodic pattern. Even with a periodic pattern (e.g., for DRAM), there are significant issues when modifying a Fourier filter in a reticle scanning process. With an inspection system such as inspection system 100, there is also a limitation to use only a collimated illumination beam for its Fourier filtration. Therefore, it does not allow the illumination optimization necessary for suppression of scattering from reticle surface roughness.
Precision, quality, and certainty of particle detection is very often compromised when using known inspection systems. With increasing demands for higher throughput and shrinking lithographic feature sizes, it is becoming increasingly important to enhance an inspection system's performance in terms of speed, smaller particle size detection, and immunity against unwanted effects.